<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
</head>

<body>

</body>
<script>
    // function fn(a) {
    //     a(function(){
    //         console.log(2);
    //     });
    // }

    // fn(function (b) {
    //     console.log(1)
    //     b();
    // })
    function sum(x,y,callback){
        var sum = x+y;
        callback(sum);
    }
    sum(1,3,function(sum){
        console.log(sum)
    })
</script>

</html>